Currently most long term evolution (LTE) networks have not deployed commercial IP Multimedia Subsystems (IMS) within their own networks. Thus voice call services are not natively supported in the LTE network, and Circuit Switched fallback (CSFB) is used as the alternative approach. However the CSFB solution requires the LTE operator to connect its core network with the 2G/3G systems either in Gateway System for Mobile Communications (GSM) or Code Division Multiple Access 1×(CDMA1×).
This requirement poses challenges for (1) the Greenfield LTE operators as the visiting operators to support inbound voice roaming service to the roamers requiring CSFB support; (2) LTE plus CDMA1× network as the visiting network to support CSFB from Global System for Mobile Communications (GSM) home operators; and (3) LTE plus GSM network as the visiting network to support CSFB from CDMA home operators.
As used herein, the term “Greenfield LTE operator” refers to the three types of operators mentioned as above. Generally, a Greenfield LTE operator is defined as an operator that has deployed an LTE network but does not have any legacy 2G/3G networks. They may deploy voice/SMS (Short Message Service) components under their IMS systems over the LTE network to offer voice and SMS services to their direct subscribers. However, due to the lack of 2G/3G networks, these Greenfield operators do not support CSFB in their own networks. Thus these Greenfield LTE operators may encounter problems when they try to offer inbound voice roaming services for their LTE roaming partners that don't deploy an IMS system.
Roamers from those home networks request voice support when they roam into such visited networks. If the roamer's device discovers that the network to be attached does not support CSFB during the network attach process, it may re-select other networks that can support CSFB and switch away from the current network. As a result, the Greenfield LTE operator loses the roamer and consequently loses the potential roaming revenue. This can be true even though for some cases the home operator may prefer to use the Greenfield LTE operator due to business and partnership considerations.